1. Field of the Invention
The present invention relates to a ball screw nut support device for converting rotational motion into linear motion or a ball screw nut support device for inversely converting linear motion into rotational motion, and an injection molding machine incorporating the ball screw nut support device. More particularly, it relates to a support construction for a plurality of ball screw nuts that individually support a load in the one-side direction of a ball screw used for injection, mold opening/closing, mold clamping, and linear driving of an ejector etc. in an electric injection molding machine.
2. Description of Related Art
Conventionally, as a driving source for a straight movement axis of an injection molding machine, oil pressure has mainly been used. In recent years, however, electrical drive has often been used because of its advantage that the work environment is improved, the electric energy efficiency is increased, and the control of the velocity, position, and the like of a working portion is facilitated. Specifically, as a driving source for injection drive of an injection screw, platen movement of mold clamping device, mold clamping, and linear movement of the ejector etc., an electric servomotor and a ball screw construction having the highest mechanical efficiency for converting the rotation of the motor into linear drive have been used.
Some electric injection driving unit for the conventional injection molding machine has one or two drive motors. In the case where one drive motor is provided, a pair of screws and nuts for converting the rotation into linear motion are provided symmetrically on both sides of the injection screw to balance the driving force, and the power is transmitted by a toothed belt for transmission or a gear train (for example, see an injection device of an injection molding machine disclosed in Japanese Patent Publication No. 8-9184 (No. 9184/1996)).
Although the above-described conventional electric injection driving unit has no problem when being used for a small-sized injection molding machine, when being used for an injection molding machine of a middle or larger size, a pressure required for injection at the time of injection is very high, so that a specially designed motor is needed to deliver high torque, and therefore it has a problem in that not only the cost is high but also the arrangement balance at the time of mounting of injection molding machine is bad.
In the injection drive of injection molding machine, when molten resin stored at the tip end of the injection screw is pushed rapidly into a mold cavity by advancing the injection screw, the injection screw requires a great pushing force. Therefore, when a ball screw is used for injection drive, the load is high in the one-side direction of straight reciprocating drive caused by the ball screw, so that the allowable maximum loading force of the ball screw is selected so as to correspond to the pushing force at the time of injection drive. Also, the loading force of ball screw is calculated by the pressure withstanding the rolling of a ball held between a screw groove and a ball nut groove, and the loading capacity in designing is calculated assuming that an even pressure is applied to working balls on the working spiral of screw.
In the case where a ball screw is used for the injection drive of a small-sized injection molding machine, even if a ball screw of a size having a loading capacity corresponding to the driving force of ball screw or a ball screw of a size exceeding the loading capacity is selected, the ball screw can be selected from commercially available standard sizes, so that the cost of ball screw device scarcely poses a problem. However, when the injection pressure increases in an injection molding machine of a middle or larger size, the size of ball screw becomes out-of-standard, so that the cost presents a big problem, and thus limit design is required.
When a high tensile force is applied in the axial direction of a ball screw shaft, the ball screw shaft elongates in its portion between a bearing on the fixed side and a ball nut, and the ball nut is also elongated or contracted by the axial force. Therefore, the screw pitch on the screw side of ball screw shifts from that on the ball nut side thereof, and thus the pressure received by the ball changes depending on the axial position.
FIG. 18 shows a conventional ball screw device disclosed in Japanese Patent Provisional Publication No. 2000-108175 (No. 108175/2000). This ball screw device is constructed so that a ball screw shaft 010 is rotated by being driven on the left-hand fixed side, not shown, and is restrained axially by a strong thrust bearing, so that a moving frame 06 connected to an injection screw by a ball screw of the ball screw shaft 010 is pushed by a ball screw nut 011 and thereby is moved in the injection direction indicated by an arrow mark. Reference numeral 59 denotes a flange-shaped pressure sensor (load cell).
In the construction of this ball screw device, as shown in a schematic view of FIG. 19-(I) and a characteristic graph of FIG. 19-(II) showing load as a function of axial nut width, the load received by a ball increases toward the working end of the ball nut 011. Since the capacity of a ball screw 050 increases, even if the length of the ball nut 011 is increased, deformation is greater at the end of the nut 011, and a tendency for the load at the working end to increase cannot be avoided. Reference numeral 04 denotes a fixed-side member that pivotally supports the ball screw shaft 010 via a bearing.
A ball screw device for injection drive of injection molding machine, which has been disclosed in Japanese Patent Provisional Publication No. 2000-185339 (No. 185339/2000), is constructed as described below. As shown in FIG. 20, a plurality of ball nuts 052, 053, which fit on a ball screw shaft 051 driven by an electric motor, are disposed in series on one ball screw shaft, and fluid pressure cylinders 054 and 055 for transmitting a thrust from the ball nuts 052 and 053 to a movable member 060 of an electric injection molding machine are connected to the ball nuts 052 and 053, respectively. Further, the cylinder chambers of the fluid pressure cylinders 054 and 055 are connected to each other by a communicating tube 056. In this ball screw device, a load applied to the ball screw shaft 051 is distributed evenly to the ball nuts 052, 053 so that the service life of the ball screw shaft 051 is prolonged.
As explained in the above-described conventional example, if an attempt is made to increase the length of the ball nut to obtain a ball screw nut of high load, the deformation in the end portion of nut increases, so that the load on the working end side tends to increase more and more. Also, the ball screw device constructed so that a plurality of ball nuts are threadedly engaged with the ball screw shaft in series, a working body is connected to each of the ball nuts via the fluid pressure cylinder, and the cylinder chambers of the fluid pressure cylinders are connected to each other by the communicating tube is capable of evenly distributing a load applied to the nut. In this ball screw device, however, the fluid may leak from the fluid pressure cylinder, or the position of ball nut may be changed by the compression of oil due to pipe volume, so that the injection screw may be incapable of keeping the proper positions of injection start and end.
An object of the present invention is to provide a ball screw device in which an even load is applied to each of a plurality of ball screw nuts without a change of attachment position of the ball screw nut with respect to a working body. Also, another object of the present invention is to provide a ball screw device in which a plurality of ball screw nuts can share a heavy load of a ball screw shaft without a change of attachment position of the ball screw nut with respect to a working body.
The present invention solves the above problems by using measures featuring the configurations of the following items:
(1) A ball screw device which converts the rotation of a ball screw shaft whose thrust direction is restrained by a fixed member into linear movement of a ball screw nut engaging threadedly with the ball screw shaft to move a working body connected to the ball screw nut, wherein the ball screw device comprises a plurality of ball screw nuts which engage threadedly with the ball screw shaft; a sensor carrying flange which attaches one of the ball screw nuts to the working body; a load sensor which is mounted on the sensor carrying flange; an annular fluid pressure piston which is attached to the ball screw nut other than the one ball screw nut; a fluid pressure cylinder, which is attached to the working body and is open on one side, having an annular groove portion in which the piston fits in a fluid-tight manner to form a fluid pressure actuator; and a fluid pressure control unit which is provided on the fluid pressure cylinder to control the fluid pressure so as to produce a piston pushing force equal to the detected load of the load sensor, so that a load is applied to the working body, the load is distributed evenly to the ball screw nuts.
(2) A ball screw device of a plurality of rows, in which a plurality of ball screw shafts whose thrust direction is restrained by a fixed member are provided in parallel, the ball screw shafts are rotated synchronously to be subjected to an even load, and a working body connected to a ball screw nut engaging threadedly with the ball screw shaft is moved linearly, wherein the ball screw device comprises a plurality of ball screw nuts which engage threadedly with each of the ball screw shafts; a sensor carrying flange which attaches one of all of the ball screw nuts to a working body; a load sensor which is mounted on the sensor carrying flange; an attachment flange which directly attaches the working body to one of the ball screw nuts on the ball screw shaft other than the ball screw shaft with which the ball screw nut having the load sensor engages; an annular fluid pressure piston which is attached to the ball screw nut other than the one ball screw nut; a plurality of fluid pressure cylinders, each of which is attached to the working body and has an annular groove portion in which the piston fits in a fluid-tight manner to form a fluid pressure actuator; a fluid pressure control unit which controls each fluid pressure of each of the cylinders so as to produce a piston pushing force equal to the detected load of the load sensor; and a pipe which transmits the fluid pressure controlled by the fluid pressure control unit to the fluid pressure cylinders, so that a load is applied to the working body, the load is distributed evenly to the ball screw nuts.
(3) The ball screw device described in the above item (2), wherein the fluid pressure sent to each of the fluid pressure cylinders is controlled so that a piston pushing force equal to the detected load of the load sensor is produced, the fluid pressure pipes to each cylinder are caused to communicate with each other so that an equal fluid pressure is applied to each of the cylinders, so that a load is applied to the working body, the load is distributed evenly to the ball screw nuts.
(4) A ball screw device which converts the rotation of a ball screw shaft whose thrust direction is restrained by a fixed member into linear movement of a ball screw nut engaging threadedly with the ball screw shaft to move a working body connected to the ball screw nut, wherein the ball screw device comprises a plurality of ball screw nuts which engage threadedly with the ball screw shaft; a load sensor which is installed on the working body or the fixed member to detect a load on the working body; an attachment flange which directly connects one of the ball screw nuts to the working body; an annular fluid pressure piston attached to the ball screw nut other than the ball screw nut connected to the attachment flange; a plurality of fluid pressure cylinders, each of which is attached to the working body and has an annular groove portion in which the piston fits in a fluid-tight manner to form a fluid pressure actuator; and a fluid pressure control unit which controls the fluid pressure of each of the fluid pressure cylinders so as to produce a piston pushing force equal to a value obtained by dividing the load detected by the load sensor of the working body by the number of ball screw nuts, so that a load is applied to the working body, the load is distributed evenly to the ball screw nuts.
(5) A ball screw device of a plurality of rows, in which a plurality of ball screw shafts whose thrust direction is restrained by a fixed member are provided in parallel, the ball screw shafts are rotated synchronously to be subjected to an even load, and a working body connected to a ball screw nut engaging threadedly with the ball screw shaft is moved linearly, wherein the ball screw device comprises a plurality of ball screw nuts which engage threadedly with each of the ball screw shafts; a load sensor which is installed on the working body or the fixed member to detect a load on the working body; an attachment flange which directly connects one of the ball screw nuts located on the same shaft of each of the ball screw shafts to the working body; an annular fluid pressure piston attached to the ball screw nut other than the ball screw nut connected to the attachment flange; a plurality of fluid pressure cylinders, each of which is attached to the working body and has an annular groove portion in which the piston fits in a fluid-tight manner to form a fluid pressure actuator; and a fluid pressure control unit which controls the fluid pressure of each of the fluid pressure cylinders so as to produce a piston pushing force equal to a value obtained by dividing the load detected by the load sensor of the working body by the total number of ball screw nuts, so that a load is applied to the working body, the load is distributed evenly to the ball screw nuts.
(6) The ball screw device described in the above item (5), wherein a fluid pressure corresponding to a value obtained by dividing the load detected by the load sensor of the working body by the total number of ball screw nuts is calculated, fluid pressure pipes to each cylinder are caused to communicate with each other so that an equal fluid pressure is applied to each of the cylinders, the working fluid pressure is controlled so as to have a value obtained by dividing the load detected by the load sensor by the total number of ball screw nuts, so that a load is applied to the working body, the load is distributed evenly to the ball screw shafts and the ball screw nuts.
(7) An injection molding machine in which a plurality of ball screw devices, each of which is provided with a plurality of ball screw nuts of a fluid pressure even sharing type described in any one of the above items (2), (3), (5) and (6) to effect straight injection drive of an injection screw of the injection molding machine, are provided in parallel to an injection screw shaft, so that all of the ball screw nuts can share the load.
The present invention provides the ball screw device for linear movement which moves the working body by using a single ball screw shaft or a plurality of ball screw shafts and ball screw nuts. In the ball screw device, a plurality of ball screw nuts are engaged threadedly with the same ball screw shaft. Also, one of the ball screw nuts on each ball screw shaft is attached to the working body via the fixed flange, the single fixed flange or one of the fixed flanges is provided with the load sensor, and other ball screw nuts serve as a fluid pressure supporting mechanism using a cylinder and a piston. A fluid pressure is controlled so that the support force of the fluid pressure supporting mechanism is equal to the pushing force detected by the load sensor. By this configuration, a large load can be distributed evenly to the ball screw nuts, and therefore the ball screw nut of a small, short and a standard size can be used, the cost can be reduced. Also, the fluid pressure supporting mechanism of the ball screw nut can cover a difference in pitch between the ball screw shaft and the ball screw nut, which achieves an effect of improving the reliability and durability of the ball screw nut. Also, since one of the ball screw nuts on each ball screw shaft is attached to the working body via the fixed flange, there is achieved an effect of maintaining the reproducibility of the original position of the ball screw nut with respect to the ball screw shaft when the working body returns (claims 1 and 2).
If the working fluid pipe for the fluid pressure supporting mechanism of the ball screw nut is made common, the fluid pressure control circuit is simple, which achieves an effect of reducing the cost (claim 3).
Also, the present invention provides the ball screw device for linear movement which moves the working body by using a single ball screw shaft or a plurality of ball screw shafts and ball screw nuts, in which the sensor for detecting all driving forces is provided on the fixed member for supporting the working body or the driving force of the working body; a plurality of ball screw nuts are engaged threadedly with the same ball screw shaft; one of the ball screw nuts on each ball screw shaft is attached to the working body via the fixed flange, and other ball screw nuts serve as a fluid pressure supporting mechanism using a cylinder and a piston; and a fluid pressure is controlled so that the support force of the fluid pressure supporting mechanism is equal to the value obtained by dividing the pushing force detected by the driving force sensor by the total number of ball screw nuts. According to this ball screw device, a large load can be distributed evenly to the ball screw nuts, and therefore the same effect as described above is achieved (claims 4 and 5).
If the working fluid pipe for the fluid pressure supporting mechanism of the ball screw nut is made common, the fluid pressure control circuit is simple, which achieves an effect of reducing the cost (claim 6).
The ball screw device in accordance with the present invention is suitably used for electric force drive that requires a large pushing force only in the one-side direction of reciprocating movement, such as injection drive (claim 7).
The present invention of another aspect solves the above problems by using measures featuring the configurations of the following items:
(8) A ball screw device which converts the rotation of a ball screw shaft whose thrust direction is restrained by a fixed member into linear movement of a ball screw nut engaging threadedly with the ball screw shaft, and has a load configuration such that the load for moving a working body connected to the ball screw nut in the one-side direction is considerably larger than the load for moving the working body in the opposite direction, wherein the ball screw device comprises a first ball screw nut provided on the working body so that an attachment flange is located on the side opposite to the heavy load direction of the ball screw shaft; an oil pressure cylinder fixedly provided on the working body; an oil pressure piston which is fitted in the oil pressure cylinder in a fluid-tight manner and is attached to the first ball screw nut; an oil pressure source which applies a predetermined oil pressure to a ring-shaped oil pressure chamber formed by the oil pressure piston and the oil pressure cylinder; and a second ball screw nut installed fixedly to the working body so that the attachment flange is located on the side opposite to the heavy load direction of the ball screw shaft, and the ring-shaped oil pressure chamber is filled with a working fluid of a predetermined pressure and holds it, and a pre-load of a predetermined percentage of the maximum load is always applied to the working body, so that at the time of the maximum load, the two ball screw nuts share the maximum load of the ball screw shaft.
(9) A ball screw device which converts the rotation of a ball screw shaft whose thrust direction is restrained by a fixed member into linear movement of a ball screw nut engaging threadedly with the ball screw shaft, and has a load configuration such that the load for moving a working body connected to the ball screw nut in the one-side direction is considerably larger than the load for moving the working body in the opposite direction, wherein the ball screw device comprises a first ball screw nut provided on the front side of the working body so that an attachment flange is located on the side opposite to the heavy load direction of the ball screw shaft; an oil pressure cylinder fixedly provided on the working body; an oil pressure piston which is fitted in the oil pressure cylinder in a fluid-tight manner and is attached to the first ball screw nut; an oil pressure source which applies a predetermined oil pressure to a ring-shaped oil pressure chamber formed by the oil pressure piston and the oil pressure cylinder; and a second ball screw nut installed fixedly to the rear side of the working body so that the attachment flange is located on the side opposite to the heavy load direction of the ball screw shaft, and the ring-shaped oil pressure chamber is filled with a working fluid of a predetermined pressure and holds it, and a pre-load of 40 to 50% of the maximum load is always applied to the working body, so that at the time of the maximum load, the two ball screw nuts share the maximum load of the ball screw shaft.
(10) A ball screw device which converts the rotation of a ball screw shaft whose thrust direction is restrained by a fixed member into linear movement of a ball screw nut engaging threadedly with the ball screw shaft, and has a load configuration such that the load for moving a working body connected to the ball screw nut in the one-side direction is considerably larger than the load for moving the working body in the opposite direction, wherein the ball screw device comprises a first ball screw nut provided on the working body so that an attachment flange is located on the side opposite to the heavy load direction of the ball screw shaft; an elastic element provided so as to be held between the working body and a ring plate attached to the first ball screw nut; and a second ball screw nut installed fixedly to the working body so that the attachment flange is located on the side opposite to the heavy load direction of the ball screw shaft, and the elastic element is installed in a pre-loaded state so that the compressive force is a predetermined percentage of the maximum load, and at the time of the maximum load, the two ball screw nuts share the maximum load of the ball screw shaft.
(11) A ball screw device which converts the rotation of a ball screw shaft whose thrust direction is restrained by a fixed member into linear movement of a ball screw nut engaging threadedly with the ball screw shaft, and has a load configuration such that the load for moving a working body connected to the ball screw nut in the one-side direction is considerably larger than the load for moving the working body in the opposite direction, wherein the ball screw device comprises a first ball screw nut provided on the front side of the working body so that an attachment flange is located on the side opposite to the heavy load direction of the ball screw shaft; an elastic element provided so as to be held between the working body and a ring plate attached to the first ball screw nut; and a second ball screw nut installed fixedly to the rear side of the working body so that the attachment flange is located on the side opposite to the heavy load direction of the ball screw shaft, and the elastic element is installed in a pre-loaded state so that the compressive force is 40 to 50% of the maximum load, and at the time of the maximum load, the two ball screw nuts share the maximum load of the ball screw shaft.
(12) The ball screw device described in the above item (9), wherein the ball screw device further comprises a load sensor installed on a member subjected to the load of the ball screw shaft; and a control valve for controlling a pressure oil sent to the ring-shaped oil pressure chamber on the basis of the detected value of the sensor, and when the detected value is xc2xd (50%) or more of the maximum load, the oil pressure chamber is filled with the pressure oil of a predetermined pressure and holds it, and 40 to 50% of the maximum load is applied to the working body, whereby at the time of the maximum load, the two ball screw nuts share the maximum load of the ball screw shaft.
(13) An injection molding machine in which a plurality of ball screw devices, each of which is provided with a plurality of ball screw nuts of a fluid pressure sharing type described in any one of the above items (8) to (12) to effect straight injection drive of an injection screw of the injection molding machine, are provided in parallel to an injection screw shaft, so that all of the ball screw nuts can share the load.
The present invention is configured as described below. In the case where the load for moving a working body in the one-side direction of the ball screw device is considerably larger than the load for moving the working body in the opposite direction, the two sets of ball screw nuts are engaged threadedly so that the attachment flange is located on the rear side with respect to the heavy load direction of the ball screw shaft. One of the ball screw nuts is installed fixedly to the working body, and the other thereof is installed via loading means (an oil pressure mechanism consisting of an oil pressure cylinder and a piston, or an elastic element) such that a pressure of a predetermined percentage of the maximum load is always applied to the working body, so that the two ball screw nuts share the maximum load at the time of the maximum load. Therefore, no complex oil pressure control is needed, and therefore only an oil pressure source for generating a fixed oil pressure and a pressure regulating valve, or only a simple elastic element and an elastic force regulating mechanism are needed. Moreover, a ball screw nut of a standard size can be used, which brings down the cost.
Also, the ball screw device of the present invention uses a system in which a load is detected by the sensor, and when the detected value is xc2xd (50%) or more of the maximum load, a pre-load of 40 to 50% of the maximum load is applied. Thereby, the life of the ball screw nut on the pre-load side can be prolonged.
Furthermore, the fluid pressure supporting mechanism or the elastic element supporting mechanism of the ball screw nut can cover an error of attachment angle between the ball screw nut and the working body, which achieves an effect of improving the reliability and durability of the ball screw nut. Also, since one of the ball screw nuts on each ball screw shaft is attached to the working body via the fixed flange, and a pre-load is always applied to between the ball screw nut and the working body, there is achieved an effect of maintaining the reproducibility of the original position of the ball screw nut with respect to the ball screw shaft when the working body returns to the original position.
The ball screw device in accordance with the present invention is suitably used for electric force drive that requires a large pushing force only in the one-side direction of reciprocating movement, such as injection drive.